Table 1.

Evaluation of human blood-brain barrier (BBB) and intestinal absorption properties (or ADMET chemical profiles) of microbial small-molecules. As indicated, most of the microbiota-associated molecules have the potential to cross the intestinal epithelial layer as well as BBB, except kynurenic acid, dopamine and phenylethanolamine.

	Microbial metabolite ligands	Intestinal Absorption		Blood-brain barrier permeability
		Absorption	Probability	Absorption	Probability
Microbiota-encoded	N-acyl serinol	Yes	0.98	Yes	0.76
	N-acyl alanine	Yes	0.9	Yes	0.93
	N-acyl lysine/ornithine	Yes	0.94	Yes	0.9
	N-oxyacyl glutamine	Yes	0.65	Yes	0.86
	N-acyl glycine/commendamide	Yes	0.74	Yes	0.84
	Phe-Val	Yes	0.91	Yes	0.59
	Tyr-Val	Yes	0.81	Yes	0.82
Microbial transformation of host molecules	Glycocholic acid	Yes	0.95	Yes	0.84
	Phenylalanocholic acid	–	–	–	–
	Tyrosocholic acid	Yes	0.93	Yes	0.71
	Leucocholic acid	–	–	–	–
Microbial transformation of dietary precursors	Acetate	Yes	0.95	Yes	0.97
	Butyrate	Yes	0.96	Yes	0.98
	Propionate	Yes	0.96	Yes	0.97
	Tryptophan	Yes	0.95	Yes	0.98
	Tryptamine	Yes	1	Yes	0.98
	Serotonin	Yes	1	Yes	0.98
	Kynurenic acid	Yes	0.98	No	0.55
	Indole acetic acid	Yes	1	Yes	0.99
	Indole propionic acid	Yes	1	Yes	0.98
	Indolelactic acid	Yes	1	Yes	0.87
	Indoxyl sulfate	Yes	1	Yes	0.87
	Indoleacrylic acid	Yes	1	Yes	0.97
	P-cresol	Yes	1	Yes	0.89
	4-ethylphenyl sulfate	Yes	0.98	Yes	0.94
	Dopamine	Yes	0.96	No	0.84
	Histamine	Yes	0.98	Yes	0.86
	Phenylalanine	Yes	0.97	Yes	0.59
	N-Cinnamoylglycine	Yes	0.97	Yes	0.93
	Hippuric acid	Yes	0.94	Yes	0.93
	Phenylethanolamine	Yes	0.98	No	0.56
	Gamma-aminobutyric acid	Yes	0.93	Yes	0.91